Heavy smoking appears to cut the chances that an embryo will successfully imbed in the wall of the womb, research suggests.
Smoking may make the uterus unreceptive
Fertility experts examined pregnancy rates in women who underwent a cycle of IVF using donated eggs.
Researchers from Portugal and Spain, found 52.2% of light smokers became pregnant at the first time of asking, compared to 34.1% of heavy smokers.
They believe that suggests tobacco makes the uterus less receptive.
The study is published online by the journal Human Reproduction.
Lead researcher Dr Sergio Soares said: "The fact that we see this result in a situation in which the oocytes (eggs) were donated by other women demonstrates that cigarette smoking negatively affects the receptiveness of the uterus independently of its effect on ovarian function."
For the study, light smokers were defined as those who smoked fewer than 10 cigarettes a day, with heavy smokers exceeding that level.
Although the pregnancy rate was much lower in heavy smokers, for those that did become pregnant the multiple pregnancy rate was much higher, with 60% of the heavy smokers expecting twins against 31% of the non-heavy smokers.
Dr Soares said it was unclear whether this was just a quirk of the study.
But he said it possible that compounds in tobacco affect the uterus in different ways in different women, impairing implantation in some and having the opposite effect in others.
"It could be that heavy smoking disrupts the stability of cells in the lining of the uterus differently in some women to others or triggers a response in the embryo itself, resulting in a reduced general pregnancy rate overall, but an increased chance of multiple pregnancy in those who do become pregnant."
Dr Soares said heavy smokers should be told that even if fertilisation takes place, they have less chance of achieving a successful pregnancy.
"Furthermore, we should also warn them of the risks of multiple births, as multiple births are less safe for mothers or babies."
IVF expert Dr Simon Fishel, director of Care Fertility Group, said there were thousands of toxins in cigarettes which could potentially upset the delicate chemical communication between the embryo and its mother.
He said there were many changes in the uterine secretions around the time of implantation.
"This is further proof of the toxic effects of smoking. Anybody who wants to get pregnant should stop smoking, and not go to smoking areas."